When refining a molten pig iron into a steel in a top-blowing oxygen converter (hereinafter simply referred to as a ("converter"), the amount of slag discharged from the converter (hereinafter referred to as "steel slag") ranges from about 100 to about 150 kg per ton of crude steel, resulting in a huge annual amount in total.
When such a steel slag discharged in a large quantity is employed as a subbase course material or an aggregate (hereinafter generally referred to as "aggregate"), said steel slag is susceptible to disintegration, because of the expansion in volume with the lapse of time under the effect of free lime (free CaO) and dicalcium silicate (2CaO.SiO.sub.2) contained in said steel slag. In addition, because a steel slag has a relatively high iron content, the aggregate produced therefrom has a higher specific gravity than natural aggregates. For these reasons, a steel slag is very rarely utilized as aggregates, but mostly disposed of as waste for reclamation at present, except for only a very small portion used as one of blast furnace burden raw materials. However, in order to dispose of these large quantities of steel slag in the form of the waste for reclamation, vast places for reclamation are required, while it is becoming increasingly difficult to find such places for reclamation. In addition, the transporation of the waste is very costly. Development of a method for effectively utilizing or efficiently disposing of a steel slag only increasing in amount is therefore one of the important tasks requiring an urgent solution in the field of steelmaking.
Under such circumstances, many studies have been made in various sectors of industry with a view to effectively utilizing a steel slag discharged from a converter, and these studies may be broadly classified into the following categories:
(1) Studies on the removal of phosphorus contained in steel slag in an attempt to increase the usage of steel slag as a raw material for blast furnace or converter operation;
(2) Studies on the utilization of a steel slag as a construction or building material; and,
(3) Studies on the use of a steel slag as a fertilizer.
All of the above-mentioned studies except for those on the removal of phosphorus contained in a steel slag are to utilize a steel slag with the original chemical composition thereof as it is without changing its original chemical composition. Depending upon the usage of a steel slag, the utilization thereof with an unchanged chemical composition results in a problem in quality because of the coexistence of useful and detrimental substances, thus preventing the steel slag from going beyond the limit of a low-quality substitute material. Therefore, only a small quantity of steel slag can be utilized when, for example, employing the steel slag as a substitute for the cement material.
In order to utilize a steel slag effectively and in a large quantity, therefore, it is necessary to remove detrimental substances and separate and recover useful substances from constituents of the steel slag, and in this connection, the following proposals have been made:
(1) A melting process of steel slag, as disclosed in Japanese Patent Provisional Publication No. 83,693/74 of Aug. 12, 1974, which comprises:
charging, into an electric melting furnace containing a molten pig iron, a molten steel slag together with bauxite as an alumina (Al.sub.2 O.sub.3) source; contacting said molten slag with said molten pig iron by stirring, while heating the contents of said furnace; transferring the Mn, P and Fe constituents contained in said molten steel slag into said molten pig iron by reducing MnO, P.sub.2 O.sub.5 and Fe.sub.2 O.sub.3 contained in said molten steel slag with carbon contained in said molten pig iron, thereby eliminating constituents detrimental to cement from said molten steel slag; discharging said molten steel slag thus treated from said melting furnace and rapidly cooling said molten steel slag for solidification; and finely pulverizing said solidified steel slag, thereby manufacturing an alumina cement from the steel slag (hereinafter referred to as the "prior art (1)").
(2) A process for recovering Fe and P constituents in a steel slag, as disclosed in Japanese Patent Provisional Publication No. 73,114/77 of June 18, 1977, which comprises:
adjusting the chemical composition of a molten steel slag contained in a vessel to a chemical composition comprising from 10 to 21 wt.% SiO.sub.2, from 2.5 to 7 wt.% Al.sub.2 O.sub.3, from 18 to 28 wt.% total Fe, and from 26 to 38 wt.% CaO and reducing the viscosity of said molten steel slag by adding iron oxides and a mineral mainly comprising SiO.sub.2 ; reducing Fe.sub.2 O.sub.3 and P.sub.2 O.sub.5 contained in said molten steel slag through reactions caused in said molten steel slag under a reducing atmosphere by the sensible heat of said molten steel slag, and as required, by a sufficient heat supplied from outside, thereby separating molten Fe from said molten steel slag and transferring P constituent contained in said molten steel slag into said molten Fe; and utilizing said steel slag thus treated and almost free of P as a source of lime and Mn for blast furnace burden (hereinafter referred to as the "prior art (2)").
(3) A process of treating a steel slag for recovering useful substances from said steel slag, as disclosed in Japanese Patent Provisional Publication No. 75,607/77 of June 24, 1977, which comprises:
charging a steel slag, together with a carbonaceous reducing agent, into a reducing furnace; and reducing said steel slag by heating at a temperature of at least 1,600.degree. C. in said reducing furnace, thereby recovering calcium carbide and ferromanganese from said steel slag (hereinafter referred to as the "prior art (3)").
(4) In a process for upgrading molten basic slags containing desired metals by removing unwanted impurities therefrom, the improvements as disclosed in the U.S. Pat. No. 3,091,524 of May 28, 1963, which comprise:
(a) contacting said slag with a molten alloy within an electric furnace and at a suitable smelting temperature;
(b) carbonaceously reducing into said alloy as much of the desired metal as is required to simultaneously reduce said unwanted impurities from said slag; and,
(c) blowing an oxygen-containing gas into the alloy to reoxydize at least a major portion of the desired metal back into the slag and retaining a major portion of impurities in the metal, whereby said slag is freed of unwanted impurities with only a small loss of the desired metal (hereinafter referred to as the "prior art (4)").
(5) A process for treating a steel slag, as disclosed in German Patent Provisional Publication No. 2,307,237 of Aug. 23, 1973, which comprises:
separating a molten steel slag into a slag and a metal through a reduction treatment in an electric furnace, whereby said slag is used as a material for cement and said metal is used as a material for ferromanganese (hereinafter referred to as the "prior art (5)").
In the prior art (1), the reduction treatment of the steel slag requires a large quantity of molten pig iron and a considerable amount of heat. In addition, in the prior art (1) in which such constituents as P and Mn contained in the steel slag are transferred into the molten pig iron, the reuse of said molten pig iron having high P and Mn constituents for steelmaking requires elimination of these constituents from said molten pig iron, thus requiring much time and costs.
In the prior art (2) in which the steel slag is subjected to a reduction treatment to separate a molten metal mainly comprising Fe from the steel slag, the P constituent in the steel slag is transferred into said molten metal, whereas the Mn constituent more or less remains in the steel slag. While, in the prior art (2), the steel slag from which the P constituent has thus been removed is used as a source of lime and Mn for blast furnace burden, no contemplation is made on the utilization of said molten metal having contained P in a large quantity.
In the prior art (3) calcium carbide and ferromanganese are recovered from the steel slag through a reduction treatment of said steel slag. However, the calcium carbide thus obtained has a low purity, and the ferromanganese thus obtained is of a low grade with an Mn content of only 10-odd wt.%, both posing quality difficulties.
The prior art (4) is as problematic as the aforementioned prior art (1) in that it is necessary to use a molten alloy in a considerable amount with a view to removing unwanted impurities.
The prior art (5) is somewhat similar to the process of the present invention presented later in that a molten steel slag is separated into a slag and a metal by reducing the steel slag in an electric furnace; the slag thus separated is used as a cement or as a material for cement; and the metal thus separated is used as ferromanganese. However, the ferromanganese obtained in accordance with the prior art (5) is inferior in quality because of the high P content. Furthermore, in the prior art (5), no provision is made to further decompose the metal thus separated to recover useful substances with a high added value.
In the prior arts (1) to (5), as mentioned above, it is proposed to subject a steel slag to a reduction treatment to separate said steel slag into a slag and a metal, and to utilize said slag and said metal for various uses, whereas no proposal is made with regard to a further treatment of the metal thus separated for decomposition and the resulting recovery of useful substances with a higher added value from said metal.